Method of manufacturing glass substrate for magnetic disk, method of manufacturing magnetic disk, and polishing apparatus of glass substrate for magnetic disk

ABSTRACT

A method of manufacturing a glass substrate for a magnetic disk having polishing accuracy on an inner circumferential end face of the substrate, and reduced thermal asperities. An inner circumferential end face of a cylindrical polishing object  12 , comprising a plurality of stacked glass substrates  20 , is polished. A plurality of polishing cloths disposed around a rotation axis of an inner circumference polishing section having the rotation axis are contacted to the inner circumferential end face  116  of the polishing object at even pressure. Then a polishing liquid is supplied between the inner circumferential end face of the polishing object and the inner circumference polishing section. Finally, the inner circumference polishing section and the polishing object are relatively rotated/moved with the rotation axis as a center, or relatively moved in a direction of the rotation axis, whereby the inner circumferential end face of the polishing object is polished.

This is a divisional of application Ser. No. 12/088,836 filed Mar. 31,2008. The entire disclosure of the prior application, application Ser.No. 12/088,836, as well as the content of Japanese Patent Application2006-269723 from which priority has been claimed in the priorapplication, is considered part of the disclosure of the accompanyingDivisional application and is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a glasssubstrate for a magnetic disk, by which an inner circumferential endface of a glass substrate is polished, a method of manufacturing amagnetic disk, and a polishing apparatus of the glass substrate for amagnetic disk.

BACKGROUND ART

Recently, with sophistication of informatization technology, informationrecording technology, particularly magnetic recording technology issignificantly improved. As a substrate for a magnetic recording mediumsuch as HDD (Hard Disk Drive) as one of media for such magneticrecording, an aluminum substrate has been widely used. However, withreduction in size or thickness of a magnetic disk, and with increase inrecording density, demand for a glass substrate is now increased, theglass substrate being high in surface flatness and high in strengthcompared with the aluminum substrate.

Moreover, with increase in recording density in magnetic recordingtechnology, a magnetic head is now changed from a thin film head to amagnetoresistive head (MR head) or a giant magnetoresistive head (GMRhead), and flying height of a magnetic head on a substrate isaccordingly reduced to about 8 nm. A magnetic head mounted with such amagnetoresistive effect element sometimes induces a thermal asperitytrouble as a unique trouble of the head.

The thermal asperity trouble is a trouble where when a magnetic head inflying in the air passes above a small convex or concave portion on amagnetic disk surface, the magnetoresistive effect element is heated dueto adiabatic compression of air or contact with the small portion,causing a read error. Therefore, for the magnetic head mounted with themagnetoresistive effect element, a magnetic disk surface is required tohave extremely high smoothness and flatness. Moreover, when a magneticlayer is formed on a glass substrate with dust or a foreign substancebeing adhered, a convex portion is inconveniently formed, therefore theglass substrate is required to be highly cleaned for completely removingthe dust or the foreign substance.

Furthermore, substrate size recently tends to be reduced in order tomount a large-capacity magnetic recording medium on a mobile device.Therefore, a 1.8-inch substrate, a 1-inch substrate, or a furthersmall-size substrate is required rather than a 3.5-inch substrate or a2.5-inch substrate in the past. When a substrate is reduced in size inthis way, an allowable dimension error is also reduced, consequentlymore precise inner-diameter processing is required.

In addition to smoothness and flatness of the magnetic disk surface,strict accuracy control is required for a dimension error in innerdiameter of a circular hole provided in a center of a magnetic disk.This is because a dimension error of an inner circumferential end faceof a magnetic disk has a direct influence on setting accuracy when themagnetic disk is fittingly set on a spindle motor of HDD. Moreover, alarge dimension error in inner diameter leads to a possibility ofinducing a mechanical error in stacking servo (writing of servoinformation into a magnetic disk) performed before the magnetic disk isassembled in a magnetic disk device such as HDD, or a possibility ofinducing bad fitting of the disk with a spindle in disk stacking. Theinner circumferential end face of a magnetic disk is small in surfacearea compared with a main surface, and when a rotational center of themagnetic disk is displaced due to the dimension error in inner diameter,it is difficult to dispose a head of HDD in a correct position on theHDD, consequently data are hardly recorded or reproduced.

Moreover, since magnetic disk is subjected to read/write of data whilerotating at high speed, it is necessary that data on the magnetic diskdo not move even during such high-speed rotation. Therefore, accuracycontrol of a dimension error in inner diameter is particularly importantfor a substrate for a magnetic disk.

Furthermore, when attention is paid on data access for HDD, a servopattern to be an index for positioning is previously written into amagnetic disk assembled in the HDD to accurately store/reproduce data ofthe magnetic disk. Such writing of the servo pattern is carried outwhile the magnetic disk is fittingly set on a device called servowriter. The magnetic disk written with the servo pattern is temporarilyseparated from the servo writer, and fittingly set on the spindle motorof HDD being a product.

In the case that the dimension error in inner diameter of a magneticdisk is large, when the magnetic disk is assembled in HDD, alignmentbetween a servo pattern and a position of a recording/reproducing headof HDD being a product is disordered, therefore recording/reproducing ofdata is still not normally performed. While a technique of adjustingalignment for correcting such a positional relationship is disclosed,the technique does not give a drastic solution for suppressing thedimension error in inner diameter.

Under such a situation, to avoid the thermal asperity trouble, it isnecessary to smoothen a surface of a magnetic disk, in addition, tosmoothen (mirror-finish) an end face of the magnetic disk. Moreover, toprevent movement of a rotation axis when the magnetic disk is fittedwith the spindle motor, an inner circumferential end face of themagnetic disk needs to be processed at high accuracy. Thus, disclosureis made on a technique of polishing the inner circumferential end faceof the magnetic disk into an end face having a certain roughness orlower by using a polishing brush having brush hair curled in ameandering pattern (for example, see Patent Document 1(JP-A-2004-155652)).

Patent document 1: JP-A-2004-155652

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

Moreover, further improvement in recording density is recently required,and further improvement in inner diameter tolerance of a magnetic diskis accordingly required. However, in a configuration that a polishingbrush or a polishing pad is used as a polishing material, there has beena limit in improving processing accuracy.

FIG. 1 shows a diagram for illustrating a first related art, which is anexplanatory diagram showing a polishing step of an inner circumferentialend face when a polishing brush 42 is used as a polishing material.Outer diameter of the polishing brush 42 needs to be formed smaller thaninner diameter of a glass substrate 20 in order to insert the brush intothe inner diameter of the glass substrate 20. To polish the innercircumferential end face, the polishing brush 42 is revolved on innercircumference of the glass substrate 20 while rotating in a directionopposite to a rotation direction of the glass substrate 20 as shown byan arrow in the figure. Furthermore, the polishing brush 42 itself isswung at low speed in a direction of a rotation axis of the polishingmaterial (stroke motion) to polish the inner circumferential end face asa whole. While the outer diameter of the polishing brush 42 is smallerthan the inner diameter of the glass substrate 20, the polishing brush42 is generally contacted to the inner circumferential end face of theglass substrate 20 at a plane 44 due to a brush configuration of thepolishing brush, as shown in FIG. 1. Consequently, the glass substrate20 is applied with a pressing force biased in one direction toward theplane 44 of the polishing brush 42.

In such a configuration using the polishing brush 42, an outercircumferential shape of the polishing brush 42 is unfixed, and when thepolishing brush 42 is contacted to the inner circumferential end face ofthe glass substrate 20, the polishing brush 42 has certain elastic forcecaused by each strand of hair of the brush, therefore tolerance of innerdiameter is increased, and the inner diameter is accordingly harder tobe controlled. Particularly, in the case of the glass substrate 20 usedfor a magnetic disk, a strong demand exists for large-volume andlow-cost production, and therefore increase in yield is necessary.However, polishing using the polishing brush 42 has a limit in controlor management of inner diameter accuracy, resulting in extremely largenumber of bad products due to fluctuation in the inner diameter.

To improve such processing accuracy, it can be further considered that arod-like polishing pad, in which at least an outer circumferentialsurface is configured by a polishing cloth, is used in place of thepolishing brush 42, and pressed to the inner circumferential end facefor polishing.

FIG. 2 shows a diagram for illustrating a second related art, which isan explanatory diagram showing a polishing step of an innercircumferential end face when a polishing pad 50 is used as a polishingmaterial. Outer diameter of the polishing pad 50 also needs to be formedsmaller than inner diameter of a glass substrate 20 in order to insertthe pad into the inner diameter of the glass substrate. The polishingpad 50 is revolved on inner circumference of the glass substrate 20while rotating in a direction opposite to a rotation direction of theglass substrate 20 as shown by an arrow in the figure. Furthermore, thepolishing pad 50 itself is swung at low speed in a direction of arotation axis (stroke motion) to polish the inner circumferential endface as a whole. While the outer diameter of the polishing pad 50 issmaller than the inner diameter of the glass substrate 20, and thepolishing pad 50 has a fixed form, the polishing pad 50 is contacted tothe inner circumferential end face at a line 52 or a point, andconsequently the glass substrate 20 is applied with a pressing forcebiased in one direction via only the line 52.

When the polishing pad 50 is used for polishing in this way, since acontact surface with the glass substrate 20 is reduced, the polishingpad 50 polishes the inner circumferential end face while revolvablymoving on the inner circumferential end face, consequently polishingtime cannot be reduced. Therefore, the inner circumferential end facehas not been able to be evenly polished due to change in pressing forceto the inner circumferential end face during polishing because of linecontact, causing a problem of increase in inner diameter circularity orconcentricity. Furthermore, since only polishing at low speed can beperformed, productivity has been hard to be improved.

Moreover, a problem occurs in the configuration of the first or secondrelated art, that is, since the polishing brush or the polishing padneeds to be revolved for polishing, an axis of the polishing brush orthe polishing pad may move during polishing the inner circumference,leading to degradation of circularity and concentricity of the innercircumferential end face after polishing the inner circumference.Therefore, it has been extremely difficult to achieve a dimension(shape) of the inner diameter to which particularly severe requirementsare recently made.

The invention was made in the light of the above difficulties existingin polishing treatment of a glass substrate, and a problem of theinvention is to provide a method of manufacturing a glass substrate fora magnetic disk, a method of manufacturing a magnetic disk, and apolishing apparatus of a glass substrate for a magnetic disk, thosebeing novel and improved, by which high processing accuracy can beobtained on an inner circumferential end face of the glass substrate,and more particularly, circularity and concentricity of the innercircumferential end face of the glass substrate after polishing theinner circumference can be significantly improved compared with those inthe past.

Means for Solving the Problems

To solve the problem, according to an aspect of the invention, a methodof manufacturing a glass substrate for a magnetic disk is provided, bywhich an inner circumferential end face of a cylindrical polishingobject is polished, the polishing object including a plurality ofdisk-like glass substrates stacked on one another, each substrate havingan inner hole formed in its center, wherein a plurality of polishingcloths disposed around a rotation axis of an inner circumferencepolishing section having the rotation axis are contacted to the innercircumferential end face of the polishing object at even pressure, thena polishing liquid is supplied between the inner circumferential endface of the polishing object and the inner circumference polishingsection, and then the inner circumference polishing section and thepolishing object are relatively rotated/moved with the rotation axis asa center, or relatively moved in a direction of the rotation axis,thereby the inner circumferential end face of the polishing object ispolished.

According to such a configuration, the polishing cloths can be pressedin face contact to the inner circumferential end face as a whole at evenpressing force. Moreover, since the rotation axis of the innercircumference polishing section need not be revolved or moved, stableinner-diameter circularity, stable concentricity, and low inner diametertolerance can be achieved. Furthermore, since the inner circumferentialend face can be polished in a manner of face contact with the polishingcloths, high polishing speed, that is, high productivity can be obtainedcompared with a case that the inner circumferential end face is polishedin a manner of point contact, line contact or the like in the past.

Here, the rotation axis of the inner circumference polishing section isa straight line that is spatially fixed to be a rotation center when theinner circumference polishing section is rotationally moved, and therotation axis is adjusted to correspond to a central axis of innercircumference of the polishing object. Rotation/movement involvesrotation about the rotation axis of the inner circumference polishingsection, namely, rotation of the inner circumference polishing sectionon its own axis, and movement of the polishing section in a directionperpendicular to the rotation axis while rotating on its own axis insuch a way. That is, the rotation/movement includes two types ofoperation of (1) rotation, and (2) movement in a rotation axis directionwhile rotating. In polishing according to the manufacturing method, theinner circumference polishing section is rotated in a manner that acenter of a circular inner hole formed in a center of a disk-like glasssubstrate corresponds to the rotation axis of the inner circumferencepolishing section. Thus, the inner circumference polishing section canpolish the inner circumference end face without any movement of theaxis. Therefore, high circularity can be achieved.

The plurality of polishing cloths may be arranged to be an even numberof polishing cloths. According to such a configuration, since thepolishing cloths are reversely formed in pairs with a core of the innercircumference polishing section between them, pressing force to theinner circumferential end face becomes uniform, and consequently smallerinner diameter circularity and smaller concentricity can be obtained.

The plurality of polishing cloths are preferably disposed so as to be inpositions reverse to one another.

The plurality of polishing cloths are preferably disposed around theinner circumference polishing section so as to be equidistant from oneanother.

A plurality of polishing cloths are disposed so as to be in positionsreverse to one another, or disposed so as to be equidistant from oneanother, thereby when the inner circumferential end face is polished, inthe case that the inner circumference polishing section is rotated, thesection can be rotated without any movement of the rotation axis. Thus,the circularity and concentricity of the inner circumferential end faceof the glass substrate can be further improved.

A configuration may be provided, in which the plurality of polishingcloths are moved in a direction perpendicular to an extending directionof the rotation axis (rotation radius direction or rotation/movementradius direction), thereby the inner circumference polishing section iscontacted by pressure to the inner circumferential end face of thepolishing object.

According to such a configuration, contact pressure between the innercircumference polishing section and the inner circumferential end faceof the polishing object can be adjusted to be an appropriate value,consequently inner diameter circularity and concentricity, which aremore stable and smaller, and low inner diameter tolerance can beachieved.

A configuration may be provided, in which a sliding surface sloped withrespect to a rotation axis direction is formed on an innercircumferential edge of the inner circumference polishing section, and adrill rod is provided, which is slidably contacted to the slidingsurface, and the polishing cloths are moved in a direction perpendicularto an extending direction of the rotation axis by a wedge effect of thedrill rod.

By providing such a slide mechanism using the sliding surface, only bysliding the drill rod in the rotation axis direction, a distance andpressing force of each polishing cloth in the direction perpendicular tothe extending direction of the rotation axis can be adjusted. That is,only by forcing the inner circumference polishing section along theinner circumferential end face, the polishing cloths can be pressed tothe inner circumferential end face. Moreover, since such slide of thedrill rod can be performed without stopping rotation of a polishingdrive section, the inner circumference polishing section can be adjustedeven during polishing the polishing object.

A configuration may be provided, in which the polishing cloths and theinner circumferential end face of the polishing object are in facecontact of 50% or more with each other. Contact area between thepolishing cloths and the inner circumferential end face of the polishingobject is made large in this way, thereby polishing speed can beincreased, and a glass substrate of which the inner diameter circularityand concentricity are better (smaller) can be obtained.

A configuration may be provided, in which an outline of each polishingcloth has a shape in accordance with the inner circumferential end faceof the polishing object. According to such a configuration, thepolishing cloths are adapted in shape to the inner circumferential endface, and thus a contact surface of each polishing cloth can be securelycontacted to the inner circumferential end face, and consequently innerdiameter circularity and concentricity, which are more stable andsmaller, and low inner diameter tolerance can be achieved.

To solve the problem, according to still another viewpoint of theinvention, a method of manufacturing a glass substrate for a magneticdisk is provided, by which an inner circumferential end face of acylindrical polishing object is polished, the polishing object includinga plurality of disk-like glass substrates stacked on one another, eachsubstrate having an inner hole formed in its center, wherein polishingcloths are pressed in face contact to the inner circumferential endface, then a polishing liquid is supplied between the innercircumferential end face of the polishing object and an innercircumference polishing section, and then the polishing cloths and theinner circumferential end face are relatively moved, thereby the innercircumferential end face of the polishing object is polished.

According to such a configuration, the polishing cloths can be pressedin face contact to the inner circumferential end face of the glasssubstrate at even pressing force, consequently high processing accuracycan be obtained on the inner circumferential end face of the glasssubstrate.

To solve the problem, according to another viewpoint of the invention, amethod of manufacturing a glass substrate for a magnetic disk isprovided, by which an inner circumferential end face of a disk-likeglass substrate is polished, the substrate having an inner hole formedin its center, wherein a plurality of polishing cloths disposed around arotation axis of an inner circumference polishing section having therotation axis are contacted by pressure to the inner circumferential endface of the disk-like glass substrate at even pressure, then a polishingliquid is supplied between the inner circumferential end face and theinner circumferential end polishing section, and then the innercircumferential end polishing section and the glass substrate arerelatively rotated/moved with the rotation axis as a center, orrelatively moved in a direction perpendicular to a main surface of theglass substrate, thereby the inner circumferential end face of thepolishing object is polished.

The invention can be applied to not only the inner circumferential endface of the cylindrical polishing object including the plurality ofdisk-like glass substrates stacked on one another, each substrate havingan inner hole formed in its center, but also the inner circumferentialend face of a disk-like glass substrate having an inner hole formed inits center, that is, the invention can be applied to a single glasssubstrate. Therefore, in the case of a single glass substrate, the innercircumferential end face can be also polished at high accuracy.

To solve the problem, according to another aspect of the invention, amethod of manufacturing a magnetic disk is provided, wherein at least amagnetic layer is formed on a surface of the glass substrate obtainedaccording to the relevant method of manufacturing a glass substrate fora magnetic disk. Thus, since inner diameter tolerance can be reducedcompared with in the past, even in the case of a magnetic disk havingincreased recording density, a read error of a signal can be prevented.

To solve the problem, according to still another aspect of theinvention, a polishing apparatus of a glass substrate for a magneticdisk is provided, which polishes an inner circumferential end face of acylindrical polishing object including a plurality of disk-like glasssubstrates stacked on one another, each substrate having an inner holeformed in its center, the apparatus having an inner circumferencepolishing section that has a rotation axis, and has a plurality ofpolishing cloths disposed around the rotation axis, and presses theplurality of polishing cloths to the inner circumferential end face ofthe polishing object at even pressure, a polishing liquid supply sectionthat supplies a polishing liquid between the polishing cloths and theinner circumferential end face of the polishing object, and a polishingdrive section that relatively rotates/moves the inner circumferencepolishing section and the polishing object with the rotation axis as acenter, or relatively moves them in a rotation axis direction so thatthe inner circumferential end face of the polishing object is polished.

According to such a configuration, as in the method of manufacturing aglass substrate for a magnetic disk, the polishing cloths can be pressedin face contact to the inner circumferential end face of the glasssubstrate at even pressing force, consequently inner diametercircularity and concentricity, which are small and stable, and low innerdiameter tolerance can be achieved.

A component corresponding to a dependant in the method of manufacturinga glass substrate for a magnetic disk, or description of the componentcan be applied to the relevant polishing apparatus of a glass substratefor a magnetic disk.

Moreover, a method of manufacturing a glass substrate for a magneticdisk according to the invention includes an inner circumferencepolishing step of polishing an inner circumferential end face of acylindrical polishing object including a plurality of disk-like glasssubstrates stacked on one another, each substrate having an inner holeformed in its center, the method being acceptably designed such that apolishing member, which has a rotation axis and polishing parts providedaround the rotation axis, is inserted into an inner hole of thepolishing object, then the polishing parts are expanded in a directionperpendicular to the rotation axis of the polishing member, thereby thepolishing parts are elastically pressed to the inner circumferential endface, and while the center of the inner hole of the glass substrate isallowed to correspond to the axis of the rod-like polishing member, atleast one of the polishing object and the polishing member is relativelymoved, so that the inner circumferential end face of the glass substrateis polished.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be further designed such that theplurality of polishing parts of the polishing member are provided inpositions reverse to one another.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that each of thepolishing parts includes a polishing cloth or a grinding stone.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the polishingparts include polishing cloths respectively, and a polishing liquidcontaining abrasive grains is supplied between the polishing cloths andthe inner circumferential end face, then at least one of the polishingobject and the polishing member is relatively moved, thereby the innercircumferential end face of the glass substrate is polished.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the polishingparts include grinding stones respectively, and a liquid coolant issupplied between the grinding stones and the inner circumferential endface, then at least one of the polishing object and the polishing memberis relatively moved, thereby the inner circumferential end face of theglass substrate is polished.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the methodfurther includes a chemical strengthening treatment step in which theglass substrate is contacted to a chemical strengthening treatmentliquid, thereby part of ions contained in the glass substrate aresubstituted by ions in the chemical strengthening treatment liquid,thereby the glass substrate is chemically strengthened, and an innercircumference polishing step is performed after the chemicalstrengthening treatment step, and the inner circumferential end face ofthe glass substrate is polished in the inner circumference polishingstep such that at least part of a compressive stress layer is remained,the compressive stress layer being formed on the inner circumferentialend face of the glass substrate by the chemical strengthening treatmentstep.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that thickness ofthe compressive stress layer formed in the chemical strengtheningtreatment step is less than 150 gm.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that removal depthis less than 5 gm in the inner circumference polishing step.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the innercircumferential end face is polished in the inner circumferencepolishing step such that circularity of an inner hole is within5_(1,1M).

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the methodfurther includes a main surface polishing step of polishing a mainsurface of the glass substrate, and the main surface is polished in themain surface polishing step such that surface roughness (Ra) is 0.2 nmor less when the surface roughness is measured by an atomic forcemicroscope.

Moreover, the method of manufacturing a glass substrate for a magneticdisk according to the invention may be designed such that the glasssubstrate has a chamfered surface between the main surface and the innercircumferential end face, and the method further includes achamfered-surface polishing step in which polishing cloths are used, thepolishing cloths being contactable to the chamfered portion over thewhole circumference of the chamfered portion at a time, and while thepolishing cloths are pressed to the chamfered portion over the wholecircumference of the chamfered portion at a time, the polishing clothsand the glass substrate are relatively moved, thereby the chamferedsurface is polished.

Advantage of the Invention

As described hereinbefore, according to the inner circumferencepolishing section of the invention, polishing cloths of the innercircumference polishing section is made in face contact to an innercircumferential end face as a whole of a polishing object, and pressingforce in such face contact can be made uniform, therefore highprocessing accuracy can be obtained, that is, inner diameter circularityand concentricity can be made small and stable, and thus inner diametertolerance can be kept low, and furthermore polishing speed can beincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram for illustrating a first related art, which showsan explanatory diagram showing a polishing step of an innercircumferential end face when a polishing brush is used as a polishingmaterial.

FIG. 2 shows a diagram for illustrating a second related art, whichshows an explanatory diagram showing a polishing step of an innercircumferential end face when a polishing pad is used as a polishingmaterial.

FIG. 3 shows an explanatory diagram showing a polishing step of an innercircumferential end face in the case that an inner circumferencepolishing section is used as a polishing material in the invention.

FIG. 4 shows a sectional diagram showing an example of an innercircumference polishing section having an expanding/contractingmechanism in a direction perpendicular to an extending direction of arotation axis, used in the invention.

FIG. 5 shows a block diagram showing a schematic configuration of achamfered-portion polishing apparatus used in the invention.

10 polishing apparatus 12 polishing object 18 polishing drive section 20glass substrate 40 nozzle (polishing liquid supply section) 110 innercircumference polishing section 112 polishing body 114 polishing cloth150 sliding surface 154 drill rod

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the invention will be describedin detail with reference to accompanying drawings. In the descriptionand the drawings, respective components having substantially the samefunction and configuration are marked with the same reference, andrepeated description of them are omitted.

A glass substrate for a magnetic disk is formed via several steps.First, a wafer is cut into a disk shape, and then opened with an innerhole to be formed into a shape of a glass substrate. Then, chamfering ismade on an outer circumferential end face and an inner circumferentialend face of the glass substrate formed by cutting, and then both the endfaces are polished. Next, a main surface of the glass substrate is alsopolished, and finally the glass substrate that has been polished issubjected to chemical strengthening treatment.

An embodiment of the invention relates to a method of manufacturing aglass substrate for a magnetic disk, and particularly relates topolishing of an inner circumferential end face of a glass substrate.Hereinafter, description is made on a method of polishing the innercircumferential end face of the glass substrate in the embodiment.

(Polishing Apparatus and Polishing Method)

FIG. 3 shows a vertical section diagram for illustrating a configurationof a polishing apparatus 10 of a glass substrate for a magnetic disk.Such a polishing apparatus 10 of a glass substrate includes a polishingobject 12, a support section 14, an inner circumference polishingsection 110, and a polishing drive section 18 to polish an innercircumferential end face of the polishing object 12.

The polishing object 12 is formed in a cylindrical shape by stacking aplurality of glass substrates 20. In each glass substrate 20, an outercircumferential end face and an inner circumferential end face arechamfered in a forming step, and for example, a sidewall portion (Tplane) 22 and a chamfered portion (C plane) 24 are formed on the innercircumferential end face as shown in an enlarged diagram in FIG. 3. Theglass substrates 20 are stacked on one another via spacers 26. Eachspacer 26 is provided for securely preventing the chamfered portions 24of the inner and outer circumferential end faces of the glass substrate20 from being insufficiently polished by a polishing brush, and forsecurely preventing breakage of the glass substrate or the like duringpolishing.

The support section 14 mainly includes a substrate case 30, a fasteningcover 32, and a rotational holding stage 34. The substrate case 30serves to accommodate the polishing object 12. Particularly, thesubstrate case 30 and the fastening cover 32 to be fitted with thesubstrate case 30 fasten the polishing object 12 via a color 36. Byfastening the polishing object 12 by the substrate case 30 and thefastening cover 32 in such a way, a layout of respective glasssubstrates 20 as the polishing object 12 can be held without beinginfluenced by rotation of the support section 14 and by rotation of aninner circumference polishing section 110 described later.

The rotational holding stage 34 fixedly holds the substrate case 30, andcan rotate/move the substrate case 30 in both forward and reversedirections. Rotation speed of such a rotational holding stage 34 can beadjusted, and appropriate rotation speed can be selected depending on apurpose of polishing.

The inner circumference polishing section 110 has a rotation axis thatis perpendicular to the glass substrates 20 as the polishing object 12,and corresponds to a central axis of the inner hole of the polishingobject 12, and acts as a rotational center of the inner circumferencepolishing section 110, and includes a polishing body 112, and aplurality of polishing cloths 114 provided on a sidewall of thepolishing body 112. Such a plurality of polishing cloths 114 may be madeof a soft polisher using suede or velour, or may be made of a hardpolisher including hard velour as material, formed resin, andpitch-impregnated suede, and arranged so as to form a part of acylindrical shape with the rotation axis as a center. The polishingcloths 114 may be arranged such that respective centers of them aresituated at approximately equal spaces to one another on an optionalcircumferential line. That is, the inner circumference polishing section110 may be configured such that the plurality of polishing cloths 114are disposed with an approximately equal interval about the centralaxis.

Outer diameter of the inner circumference polishing section 110 isformed to have a curved surface along an inner diametrical shape of theglass substrate 20, which fits with a circumferential surface 116 ofinner diameter of the glass substrate 20. The inner circumferencepolishing section 110 is contacted to an inner circumferential end faceof the polishing object 12 at even pressure, then a polishing liquid issupplied between the inner circumferential end face of the polishingobject 12 and the inner circumference polishing section 110, and thenthe inner circumference polishing section 110 is rotated/moved with therotation axis as a center in a direction of an arrow shown above theinner circumference polishing section 110 in FIG. 3, so that thepolishing object 12 is polished. While rotation about the rotation axisof the inner circumference polishing section is referred to asrotation/movement here, the rotation/movement is not limited to such amotion, and includes a motion of moving in a rotation axis directionwhile rotating in such a way.

In this case, the glass substrate 20 may not be fixed and rotated on thesupport section 14. This is because since the inner circumference of theglass substrate 20 fits with the outer circumference of the innercircumference polishing section 110, sufficient polishing speed can beobtained only by rotation/movement of the inner circumference polishingsection 110. However, it is not intended to avoid polishing whilerotating the glass substrate 20 in a direction opposite to therotational direction of the inner circumference polishing section 110.Specifically, for example, it is acceptable that the inner circumferencepolishing section 110 is fixed, and only the glass substrate 20 isrotated, or conversely, the glass substrate 20 is fixed, and only theinner circumference polishing section 110 is rotated, or both arerotated relatively to each other.

Moreover, the inner circumference polishing section 110 may be swung atlow speed in a rotation axis direction in the inner hole with respect tothe glass substrate 20 (stroke motion) so as to polish the innercircumferential end face as a whole of the polishing object 12.

Since the polishing cloths 114 of the inner circumference polishingsection 110 are provided correspondingly to the inner diameter of theglass substrate 20, that is, since the cloths are formed on a curvedsurface having the same radius as that of an inner circumferentialcurved surface of the glass substrate 20, the polishing cloths 114 ofthe inner circumference polishing section 110 can be made in facecontact to the inner circumferential end face of the glass substrate 20,and can be pressed to the inner circumferential end face with even andcertain pressing force. In this way, the inner circumferential end facecan be smoothly polished, and consequently inner diameter circularityand concentricity, those being small and stable, and low inner diametertolerance can be achieved. Moreover, since the inner circumferencepolishing section 110 is in face contact to the glass substrate 20, unitpressing force applied to each polishing cloth 114 is low, and thuspolishing heat can be suppressed, and consequently degradation in theinner circumferential end face of the glass substrate 20 can beprevented.

Here, the polishing cloths 114 and the inner circumferential end face ofthe glass substrate 20 are desirably in face contact of 50% or more toeach other, and more desirably 60% or more to each other. Byestablishing such face contact of 50% or 60% or more, when the innercircumference polishing section 110 is rotated/moved, the polishingsection can be stably operated, in addition, polishing speed can beincreased by increasing percent of face contact. By increasing contactarea between the polishing cloths 114 and the inner circumferential endface of the polishing object in this way, a glass substrate havingbetter inner diameter circularity and better concentricity can beobtained.

However, when the number of the polishing cloths 114 of the innercircumference polishing section 110 is an odd number, pressing force tothe inner circumferential end face is sometimes biased. Therefore, it isdesirable that the number of such polishing cloths 114 is an evennumber, and polishing cloths 114 in pair are disposed in opposedpositions of the polishing cloths 114. In FIG. 3, six polishing cloths114 are arranged, and respective two pairs of polishing cloths 114 arereversely formed with a core of the inner circumference polishingsection 110 between them. Thus, pressing force to the innercircumferential end face is uniform, and consequently the inner diametercircularity and concentricity can be more reduced.

While it was described that the outer diameter of the innercircumference polishing section 110 was corresponding to the innerdiameter of the glass substrate 20 in the embodiment, when the outerdiameter of the inner circumference polishing section 110 does notcorrespond to the inner diameter of the glass substrate 20, the outerdiameter of the inner circumference polishing section 110 needs to beadjusted. The inner circumference polishing section 110 in theembodiment may be contacted by pressure to the inner circumferential endface of the glass substrate 20 by moving the plurality of polishingcloths 114 in a direction perpendicular to an extending direction of therotation axis, that is, by expanding/contracting the inner circumferencepolishing section 110.

When the inner circumference polishing section 110 is inserted into orextracted from the glass substrate 20, the outer diameter of the innercircumference polishing section 110 is temporarily contracted in orderto prevent the inner circumference of the glass substrate 20 from beingdamaged. Hereinafter, a configuration of expanding/contracting the innercircumference polishing section 110 in this way is described in detail.

FIG. 4 shows a sectional diagram showing an example of the innercircumference polishing section 110 having an expanding/contractingmechanism in a direction perpendicular to the extending direction of therotation axis. A polishing body 112 of such an inner circumferencepolishing section 110 is configured by an interlocking part 152, and adrill rod 154 that is slidably contacted to a tapered sliding surface150 formed obliquely with respect to the rotation axis direction on aninner circumferential edge of the interlocking part 152. Due to a wedgeeffect on the sliding surface 150, the interlocking part 152 is operatedwith being interlocked with the drill rod 154, and displacement of thedrill rod 154 in the rotation axis direction is converted intodisplacement of the interlocking part 152 in the direction perpendicularto the extending direction of the rotation axis. For example, in thecase of the inner circumference polishing section 110 of FIG. 4, whenthe drill rod 154 is slid downward in the rotation axis direction in thefigure, the interlocking part 152 is transferred outward in thedirection perpendicular to the extending direction of the rotation axis,so that the inner circumference polishing section 110 is expanded. Whenthe drill rod 154 is slid upward in the rotation axis direction in thefigure, the interlocking part 152 is transferred inward in the directionperpendicular to the extending direction of the rotation axis, so thatthe inner circumference polishing section 110 is contracted.

By providing such a slide mechanism using the sliding surface 150, onlyby sliding the drill rod 154 in the rotation axis direction, pressingforce to the inner circumferential end face of the glass substrate 20 bythe inner circumference polishing section 110 can be adjusted to have aneven and appropriate value, that is, the inner circumference polishingsection can be elastically pressed to the glass substrate 20,consequently inner diameter circularity and concentricity, which aremore stable and smaller, and low inner diameter tolerance can beachieved. Moreover, since such a slide mechanism can be operated withoutstopping rotation of the polishing drive section, the innercircumference polishing section 110 can be adjusted even duringpolishing the polishing object.

In such a polishing method, since the rotation axis of the innercircumference polishing section need not be revolved or moved, polishingspeed can be remarkably increased, and consequently productivity can beimproved.

The expanding/contracting mechanism of the inner circumference polishingsection 110 in the direction perpendicular to the extending direction ofthe rotation axis is not limited to the mechanism in the above case, andvarious mechanisms can be used, including an expanding/contractingmechanism using atmospheric pressure or oil pressure, and a mechanism ofconverting displacement of a rotation axis into displacement in thedirection perpendicular to the extending direction of the rotation axislike a folding umbrella.

The polishing drive section 18 is connected to the rotation axis of theinner circumference polishing section 110, and can rotate the innercircumference polishing section 110 in both forward and reversedirections, and can be freely moved in the direction perpendicular tothe extending direction of the rotation axis. At a side opposite to thepolishing drive section 18, even a bearing 38 for fixing the rotationaxis is provided via the inner circumference polishing section 110. Asthe bearing 38, various bearings can be used, including a bearing, ballbearing, roller bearing, and sliding bearing. Furthermore, the polishingdrive section 18 is formed freely movably even in the rotation axisdirection, so that it can vertically reciprocate the inner circumferencepolishing section 110 in the rotation axis direction. Moreover, in theembodiment, since it is only necessary that the inner circumferencepolishing section 110 and the polishing object are relatively rotated ormoved, it is acceptable that the polishing drive section 18 is connectedto the polishing object, and the polishing object is rotated or moved,thereby the inner circumferential end face of the polishing object ispolished.

When a rotation direction of the polishing drive section 18 is fixed, arotation direction of the support section 14 is correspondinglydetermined so as to be an opposite direction (relative direction) to therotation direction of the inner circumference polishing section 110.Therefore, in FIG. 3, when the polishing drive section 18 is rotated ina CW direction, the support section 14 is rotated in a CCW direction.When the sections are rotated in opposite directions to each other inthis way, relative angular velocity of polishing corresponds to sum ofangular velocities of both the sections. A lower view of FIG. 3 shows acondition where the polishing body 112 and the polishing cloths 114 areslightly rotated in the CW direction compared with an upper view of FIG.3.

A nozzle 40 as a polishing liquid supply section is provided close to acontact between the polishing object 12 and the inner circumferencepolishing section 110, so that a polishing liquid is supplied. When thepolishing is performed, the polishing is preferably performed whilesupplying a polishing liquid containing abrasive grains from the nozzle40. As the abrasive grains, typical abrasive grains such as grains ofalumina, cerium oxide, and colloidal silica can be used depending on ashape of an objective end face. As a dispersion medium for dispersingthe abrasive grains, which is not particularly limited, water ispreferable in the light of cost, but any dispersion medium can be used,if it is typically used for polishing. Moreover, for the nozzle 40,various aspects can be used, for example, each of aspects can be used,including spray, blowing, water discharge, and coating using water flow,shower, water drops and the like.

Regarding a way of supplying the polishing liquid, for example,polishing may be performed while the polishing liquid is continuouslysupplied, or may be performed while the polishing liquid isintermittently supplied.

In a glass substrate subjected to inner-circumferential end facepolishing (inner circumference polishing) according to the invention,the inner diameter circularity and the inner diameter tolerance can besignificantly improved compared with in the past.

Specifically, the inner diameter circularity of a glass substrateobtained by performing the inner circumference polishing can be made tobe 5 pm or less, preferably 3 p.m or less, and more preferably 2 p.m orless. Moreover, the inner diameter tolerance can be made to be 10₁·1 mor less, preferably 5 1 AM or less, and more preferably 2 pm or less.

As the inner-circumferential end face polishing, for example, shapetransferring processing can be used instead of shape copying processingsuch as polishing brush processing. Thus, the inner diameter tolerancecan be further reduced. Moreover, in the inner-circumferential end facepolishing, since polishing can be performed while a rotation axis of theinner circumference polishing section 110 is allowed to correspond to acentral axis of a stacked body of glass substrates (polishing object12), the inner diameter circularity can be further improved.

Moreover, the invention can be applied to not only the innercircumferential end face of the cylindrical polishing object 12 in whichthe plurality of glass substrates 20 are stacked, but also an innercircumferential end face of a disk-like glass substrate 20 in which aninner hole is formed in the center, that is, the invention can be alsoapplied to a single glass substrate 20. Therefore, even in a singleglass substrate, an inner circumferential end face can be polishedhighly accurately.

While description was made on an example of polishing the innercircumferential end face of the cylindrical polishing object 12 in whichthe plurality of glass substrates 20 are stacked in the abovedescription, for example, the inner circumferential end face of oneglass substrate may be polished as a polishing object by using the abovemethod. According to the above method, since circularity can be improvedcompared with in the past, a glass substrate for a magnetic diskpreferably used for a magnetic disk can be obtained.

Moreover, while the support section 14 is fixed, and the innercircumference polishing section 110 is rotated for polishing in theembodiment, such a case is not restrictive, and it is possible that theinner circumference polishing section 110 is fixed, and the supportsection 14 is rotated, or both the sections are rotated in opposite(relative) directions to each other.

Moreover, a glass substrate 20 is provided, of which the innercircumferential end face has a shape being formed flat by the abovepolishing method. The sidewall portion 22 of a glass substrate 20, whichis polished by the brush polishing or the pad polishing in the past, isformed to have a rounded convex shape. However, in a polishing methodaccording to the embodiment, since the sidewall portion 22 is polishedin face contact with the polishing cloths at even pressure, flatness ofthe sidewall portion 22 can be secured, and even if it is used for amagnetic disk for HDD, fitness with a rotation spindle axis can beimproved, and thus dust is reduced, so that improvement in reliabilitycan be achieved.

Moreover, a method of manufacturing a glass substrate for a magneticdisk according to the invention, which includes an inner circumferencepolishing step of polishing an inner circumferential end face as aninner-hole surface of a disk-like glass substrate having an inner holeformed in its center, may be designed such that a polishing member isinserted into the inner hole of the glass substrate, then polishingparts of the polishing member are expanded in a direction perpendicularto an axial direction of the polishing member, thereby the polishingparts are elastically pressed to the inner circumferential end face, andwhile the center of the inner hole of the glass substrate is allowed tocorrespond to an axis of the rod-like polishing member, at least one ofthe glass substrate and the polishing member is relatively moved, sothat the inner circumferential end face of the glass substrate ispolished.

Moreover, a method of manufacturing a glass substrate for a magneticdisk, which includes an inner circumference polishing step of polishingan inner circumferential end face of a cylindrical polishing object inwhich a plurality of disk-like glass substrates, each having an innerhole formed in its center, are stacked on one another may be designedsuch that a rod-like polishing member is inserted into an inner hole ofa polishing object, then polishing parts of the polishing member areexpanded in a direction perpendicular to an axial direction of thepolishing member, thereby the polishing parts are elastically pressed tothe inner circumferential end face, and while the center of the innerhole of the glass substrate is allowed to correspond to an axis of therod-like polishing member, at least one of the glass substrate and thepolishing member is relatively moved, so that the inner circumferentialend face of the glass substrate is polished.

(Polishing of Chamfered Portion (Chamfered Surface Polishing Step))

It is also possible that after the sidewall portion (T surface) 22 ofthe inner circumferential end face of the glass substrate 20 is polishedby the above polishing method, the chamfered portion (chamfered surface)(C surface) 24 is polished. Such a chamfered portion 24 may be polishedby brush polishing using the polishing brush 42, or may be polishedusing the following polishing method.

In the embodiment, polishing cloths are used, which are contactable tothe chamfered portion 24 at one side of the inner hole formed in thecenter of the glass substrate 20 over the whole circumference of thechamfered portion 24 at a time, and while the polishing cloths arepressed to the chamfered portion of the inner hole, the polishing clothsand the glass substrate are relatively moved so that the chamferedportion 24 is polished. Here, “relative movement” may refer to a motionthat one of the polishing cloths and the glass substrate 20 is driven,or a motion that both are driven. The polishing method of the chamferedportion is performed by sheet-feed polishing in which glass substratesare polished one by one, rather than batch polishing (batch processing)in which the stacked glass substrates 20 are polished. Hereinafter, thepolishing method is described in detail using a chamfered-portionpolishing apparatus.

FIG. 5 shows a block diagram showing a schematic configuration of achamfered-portion polishing apparatus. Such a chamfered-portionpolishing apparatus is configured by a substrate support section 200 anda polishing cloth support section 210. The chamfered portions 24 of theglass substrate 20 as an object of polishing are provided at both endsof the inner hole at a predetermined angle (for example, 45°) withrespect to a main surface of the glass substrate 20.

The substrate support section 200 includes a holder 220 for holding theglass substrate 20, an arm 224 being fixedly connected to the holder 220in a swingable manner, and a torque converter 222 that connects betweenthe holder 220 and the arm 224 in a freely rotatable manner with apredetermined load.

The polishing cloth support section 210 rotatably supports a sphericalpolishing cloth 232 having a spherical portion 230 at its end. As amaterial of the spherical polishing cloth 232, foamed resin such aspolyurethane can be used. The spherical polishing cloth 232 isrotationally driven by a support shaft 236 transmitted with power of amotor 234. The spherical portion 230 of the spherical polishing cloth232 presses a chamfered portion 24 at one side of the inner hole of theglass substrate 20 so as to be contacted to the chamfered portion 24over the whole circumference of the chamfered portion 24 at a time. Whenpolishing is performed, relative positions of the rotational axis of theglass substrate 20 and a rotational axis of the spherical polishingcloth 232 may be changed by swinging the arm 224.

The chamfered-portion polishing apparatus is used to polish only thechamfered portion 24, thereby the chamfered portion 24 can besufficiently mirror-polished with a small removal depth. Therefore, whena magnetic disk is produced using the glass substrate 20, occurrence ofcorrosion (precipitation of cobalt or sodium into a surface of themagnetic disk) from the chamfered portion 24 can be prevented. Moreover,since removal depth is small, processing time is reduced, consequentlyproductivity can be improved. Furthermore, since the spherical portion230 does not polish the side face 22, inner-diameter circularity andprocessing accuracy of the side face 22 are not affected by thedescribed processing method of the inner circumferential end face,consequently reduction in the circularity and accuracy due to polishingof the chamfered portion 24 may not occur.

(Other Modes)

In the description, description is made on a configuration where thepolishing cloths 114, that is, the polishing pads are used for the innercircumference polishing section 110, and the abrasive grains being freeabrasive grains are used to polish the inner circumferential end face ofthe glass substrate.

However, the invention is not limited to the above, and for example, itis acceptable that polishing stones being fixed abrasive grains are usedinstead of the polishing cloths 114 of the inner circumference polishingsection 110, and the inner circumferential end face is polished whilecoolant is supplied to the inner circumferential end face. Moreover, itis acceptable that while coolant is supplied to an inner circumferentialend face, the inner circumferential end face is polished with acircumference polishing section 110 including the fixed grinding stones(fixed abrasive grains), then while free abrasive grains are supplied tothe inner circumferential end face, the inner circumferential end faceis polished with a circumference polishing section 110 including thepolishing cloths 114. In this way, the inner circumferential end face ispolished using the fixed abrasive grains, then it is polished using thefree abrasive grains, which makes it possible to reduce a load appliedto the polishing cloths.

(End Face Polishing After Strengthening)

Next, description is made on a step that a glass substrate is subjectedto chemical strengthening treatment, then the inner circumferential endface is polished.

A compressive stress layer is formed on a surface of the glass substratethrough the chemical strengthening treatment. While thickness of thecompressive stress layer to be formed is different depending onthickness of the glass substrate, for example, in the case of the glasssubstrate in a size of 2.5-inch disk or 1.8-inch disk, which is largelyused as a glass substrate for a magnetic disk, thickness of the glasssubstrate is 0.5 to 1.0 mm, and preferable depth (thickness) of thecompressive stress layer in this case is 100 to 200 gm. The compressivestress layer is formed not only on a main surface of the glasssubstrate, but also on an inner circumferential end face (including achamfered surface).

When the glass substrate is subjected to chemical strengtheningtreatment, a dimension of inner diameter of the substrate is changed.Moreover, the inner diameter dimension is changed depending on acomposition of a chemical strengthening treatment liquid, and on achemical strengthening treatment condition.

In this way, the inner diameter dimension of the glass substrate ischanged through the chemical strengthening treatment. However, the glasssubstrate is required to have strict dimension accuracy (inner diametercircularity and inner diameter tolerance) as described before. Thus, theinner-circumferential end face polishing is performed after the chemicalstrengthening treatment step, and thereby a glass substrate havingexcellent dimension accuracy can be manufactured with strength beingincreased through the chemical strengthening treatment.

Therefore, when the inner-circumferential end face polishing isperformed after the chemical strengthening treatment step has beenperformed, the removal amount of the compressive stress layer formed onthe substrate end face needs to be reduced to the utmost, and dimensionaccuracy needs to be improved. To satisfy this condition, specifically,the removal amount in the inner-circumferential end face polishing ispreferably 5 pm or less, and more preferably 3 pm or less.

After the inner-circumferential end face polishing, thickness of thecompressive stress layer formed on the inner-circumferential end face ofthe glass substrate is preferably 50 pm or more, more preferably 100 pmor more, and further more preferably 150 gm or more. A dimension and ashape to be required are as described before.

(Main Surface Polishing)

By performing inner circumference polishing of the invention, innerdiameter circularity and inner diameter tolerance of a glass substratecan be improved. Thus, a glass substrate for a magnetic disk thatenables higher recording density can be provided. To achieve higherrecording density, roughness of the main surface needs to be decreasedin addition to improving a parameter on the inner diameter of the glasssubstrate. Specifically, for example, when surface roughness (Ra) of aglass substrate for a magnetic disk is measured by an atomic forcemicroscope, the surface roughness (Ra) is preferably 0.2 nm or less, andmore preferably 0.1 nm or less.

Hereinafter, an example using the described polishing method orpolishing apparatus is described.

Example 1

In the example, a glass substrate for a magnetic disk and a magneticdisk were manufactured through the following steps. Particularly, in anend face polishing step, the polishing method according to theembodiment is used.

(1) Shaping Step and First Lapping Step

First, melted alminosilicate glass was molded into a disk shape bydirect press using an upper mold, a lower mold, and a sleeve mold, sothat amorphous plate-like glass was obtained. As the alminosilicateglass, glass for chemical strengthening was used. In addition to directpress, the disk-like glass substrate for a magnetic disk may be obtainedby cutting sheet glass, which is formed by a down draw method or floatmethod, by a grinding stone. As the alminosilicate glass, chemicalstrengthening glass was used, which mainly contains Si0₂ of 58 to 75wt°/0, Al₂0₃ of 5 to 23 wt %, Li₂0 of 3 to 10 wt %, and Na₂0 of 4 to 13wt %.

Next, both main surfaces of the plate-like glass were subjected tolapping to be formed into a disk-shaped glass base material. The lappingwas performed using alumina-based free abrasive grains by a double-sidelapping apparatus using a planetary gear mechanism. Specifically,lapping boards were pressed to both sides of the plate-like glass fromabove and below, and while an abrasive liquid containing free abrasivegrains was supplied onto the main surfaces of the plate-like glass, thelapping boards and the plate-like glass were relatively moved to eachother to perform lapping. A glass base material of which the mainsurfaces were flat was obtained through the lapping.

(2) Cutting Step (Coring and Forming)

Next, the glass base material was cut using a diamond cutter, and adisk-shaped glass substrate was cut from the glass base material.

Next, a circular hole was formed in a central portion of the glasssubstrate using a cylindrical diamond drill, so that a donut-shapedglass substrate was formed (coring). Then, an inner circumferential endface and an outer circumferential end face were ground by diamondgrinding stones so as to be subjected to predetermined chamferingprocessing (forming).

(3) Second Lapping Step

Next, both main surfaces of the obtained glass substrate were subjectedto second lapping as in the first lapping step. By performing the secondlapping step, a fine irregular shape, which was formed on each mainsurface in the cutting step or the end face polishing step as aprecedent step, can be removed beforehand, consequently a subsequentpolishing step for the main surfaces can be completed in a short time.

(4) End Face Polishing Step

Next, an end face of the glass substrate was mirror-polished by usingthe described polishing apparatus and polishing method according to theembodiment. In this case, a slurry (free abrasive grains) containingcerium-oxide abrasive grains was used for abrasive grains. Then, theglass substrate that has been subjected to the end face polishing stepwas rinsed. Through the end face polishing step, each of the end facesof the glass substrate was processed into a mirror surface by whichgeneration of dust such as particles was able to be prevented.

In the end face polishing step, while glass substrates are superposed onone another, end faces thereof are polished, and to avoid the mainsurfaces of the glass substrate from being scratched at that time, theend face polishing step may be performed before a first polishing stepdescribed later, or may be performed before and after a second polishingstep.

In such an end face polishing step, the polishing cloths of thepolishing section are made in face contact to the inner circumferentialend face of the polishing object, and pressing force in such facecontact is made uniform, thereby high processing accuracy can beobtained. Evaluation results on processing accuracy in the embodimentcompared with that in a prior-art polishing method are described below.

[Evaluation]

The polishing method according to the example and a polishing method inthe past were used to evaluate polishing accuracy and polishing speed ofthe inner circumferential end face of the glass substrate.

In the case that an inner circumferential end face was polished while itwas in face contact with polishing cloths including foamed polyurethane,polishing speed was 74 μm/min. On the other hand, polishing speed was0.7 μm/min in a prior-art example (comparative example) in which aninner circumferential end face was polished as in the example using abrush. From this result, it is known that when the polishing accordingto the example is performed, polishing speed is high compared with aprior art, in other words, removal depth per unit time is large.

Next, inner diameter circularity was compared between the example andthe comparative example. In the case of the example, deviation from atarget value was about 0.35 to 0.65 μm. Moreover, inner diametercircularity was 2 μm in the example. On the other hand, in the case ofpolishing using the brush, deviation from a target value had a largerange, about 0.7 to 6.7 μm with an average of 1.47 μm. In thecomparative example, inner diameter circularity was 10 μm. In the casethat a rod-like polishing pad as the prior-art example (comparativeexample) was used, and the inner circumferential end face was polishedwhile the rod-like polishing pad was revolved, as a result, deviationfrom a target value had a range of about 1.1 to 4.0 μm with an averageof 2.14 μm. In this case, inner diameter circularity was 9 μm. Moreover,in the case of the example, even if removal depth was changed, a rangeof variation from a target value was kept within the above range.

Next, inner diameter tolerance in each of the example and thecomparative example is shown in Table 1. Removal depth in the innercircumference polishing step was 5 μm or less. The number of glasssubstrate samples was 10,000.

TABLE 1 Ratio of values within target value range Target value Toleranceof ±25 μm Tolerance of ±10 μm Example  100%  100% Comparative example97.9% 95.6%

That is, by using the manufacturing method according to the invention,inner diameter tolerance can be securely controlled to be 25 μm or less,and furthermore, 10 μm or less.

From the results, it can be understood that when polishing according tothe example is performed, polishing can be performed without variationin inner diameter circularity unlike the prior art.

Moreover, polishing according to the example was performed, and changein removal depth with respect to processing time was measured, as aresult, the removal depth was linearly increased with respect toprocessing time. Therefore, the inner circumference can be easilyprocessed in an optional removal depth only by adjusting the processingtime, and consequently a dimension of the removal depth is easilymanaged. Moreover, since reproducibility of the removal depth withrespect to processing time is high, sizing can be performed stably andaccurately.

In this way, according to the polishing apparatus and the polishingmethod according to the embodiment, high processing accuracy can beachieved, that is, inner diameter circularity or concentricity can bemade small and stable, and inner diameter tolerance can be kept low.

(5) Main Surface Polishing Step

As a main surface polishing step, a first polishing step was firstperformed. The first polishing step is performed mainly for removingscratches or strain remained on a main surface in the lapping step. Inthe first polishing step, a double-side polishing apparatus having aplanetary gear mechanism was used to polish the main surface by using ahard resin polisher. Cerium oxide abrasive grains were used for apolishing liquid.

More specifically, a preliminary polishing step was performed using apolishing apparatus that was able to polish both main surfaces of eachof 100 to 200 glass substrates at a time. A polishing pad was used, inwhich grains of zirconium oxide and cerium oxide were containedbeforehand.

A polishing liquid in the first polishing step was prepared by mixingcerium oxide abrasive grains having an average grain diameter of 1.1 μminto water. Abrasive grains having a grain diameter of more than 4 μmwere removed beforehand. As a result of measuring the polishing liquid,the abrasive grains contained in the polishing liquid had a maximumdiameter value of 3.5 μm, an average diameter value of 1.1 μm, and D50value of 1.1 μm.

Besides, a load applied to the glass substrate was 80 to 100 g/cm², andremoval thickness of a surface portion of the glass substrate was 20 to40 μm.

The glass substrate that had been subjected to the first polishing stepwas sequentially dipped in respective cleaning tanks of neutraldetergent, pure water, and IPA (isopropyl alcohol), and thus washed.

Next, a second polishing step was performed as the main surfacepolishing step. The second polishing step is performed for finishing themain surfaces into mirror surfaces. In the second polishing step, themain surfaces were polished into mirror surfaces using a soft foamedresin polisher by a double-side polishing apparatus having a planetarygear mechanism. Colloidal silica grains were used as abrasive grainscontained in a polishing liquid.

More particularly, a mirror polishing step was performed using apolishing apparatus in a planetary gear type, which was able to polishboth main surfaces of each of 100 to 200 glass substrates at a time. Asa polishing pad, a soft polisher was used.

A polishing liquid in the second polishing step was prepared by addingsulfuric acid and tartaric acid into ultrapure water, and further addingcolloidal silica grains having a diameter of 40 nm into the ultrapurewater. In this preparation, concentration of the sulfuric acid in thepolishing liquid was 0.15 wt %, and a pH value of the polishing liquidwas 2.0 or less. Concentration of the tartaric acid was 0.8 wt %, andthe content of the colloidal silica grains was 10 wt %.

In the second polishing step, the pH value of the polishing liquid wasnot varied, and able to be kept to be approximately constant. In theexample, the polishing liquid supplied on the surface of the glasssubstrate was collected by using a drain, then cleaned by removingforeign substances by a mesh filter, and then resupplied onto the glasssubstrate to be reused.

Polishing speed in the second polishing step was 0.25 μm/min, from whichit was known that advantageous polishing speed was able to be achievedat the above condition. The polishing speed was obtained by dividingreduction amount (processing removal depth) of thickness of a glasssubstrate necessary for finishing a surface into a predetermined mirrorsurface by required polishing time.

The glass substrate that had been subjected to the second polishing stepwas sequentially dipped in respective cleaning tanks of neutraldetergent, pure water, and IPA (isopropyl alcohol), and thus washed.Each cleaning tank was applied with an ultrasonic wave.

A surface of the glass substrate after cleaning was observed by AFM(Nanoscope manufactured by Digital Instruments Inc.) (measurement in arectangular area of 5 μm×5 μm), as a result, maximum peak height (Rmax)was 1.5 nm, and arithmetic mean roughness (Ra) was 0.15 nm. Adhesion ofthe colloidal silica abrasive grains was not confirmed. Moreover, anyforeign substance such as stainless steel or iron was not found.Increase in roughness of the substrate surface was not found betweenbefore and after the cleaning.

(6) Chemical Strengthening Step

Next, the glass substrate that had been subjected to the lapping stepand the polishing step was subjected to chemical strengthening. Thechemical strengthening was performed through a process that a chemicalstrengthening solution including potassium nitrate (60%) and sodiumnitrate (40%) being mixed to each other was prepared, and the chemicalstrengthening solution was previously heated to 400° C., and the cleanedglass substrate was preheated to 300° C., and dipped for about 3 hoursin the chemical strengthening solution. The dipping was performed in amanner that a plurality of glass substrates were accommodated in aholder such that the glass substrates were held at their end facesrespectively in order to chemically strengthen the whole surface of eachglass substrate.

The glass substrate is subjected to dipping treatment in the chemicalstrengthening solution in this way, thereby lithium ions and sodium ionsin a surface of the glass substrate are substituted by sodium ions andpotassium ions in the chemical strengthening solution respectively, sothat the glass substrate strengthened. Thickness of a compressive stresslayer formed in the surface of the glass substrate was about 100 μm to200 μm.

The glass substrate that had been subjected to chemical strengtheningwas dipped in a water tank at 20° C. and thus rapidly cooled, and keptfor about 10 min therein. The glass substrate that had been subjected torapid cooling was dipped in concentrated sulfuric acid heated to about40° C. so as to be cleaned. Furthermore, the glass substrate that hadbeen subjected to sulfuric acid cleaning was sequentially dipped inrespective cleaning tanks of pure water and IPA (isopropyl alcohol), andthus washed. Each cleaning tank was applied with an ultrasonic wave.

As described above, the first lapping step, cutting step, end facepolishing step, second lapping step, first and second polishing steps,fine cleaning, and chemical strengthening step were performed, so that aglass substrate for a magnetic disk being flat, smooth, and highly stiffwas obtained.

(7) Fine Cleaning Step

Next, fine cleaning of the glass substrate for a magnetic disk wasperformed. The fine cleaning is performed to remove the residue of apolishing agent or a foreign iron-based contamination or the like to bea cause of head crush or a thermal asperity trouble so that a glasssubstrate having a smooth and clean surface is obtained. In the finecleaning step, a water rinse cleaning step and an IPA cleaning step wereperformed after alkaline solution cleaning.

(8) Magnetic Disk Manufacturing Step

Next, for both sides of the glass substrate obtained through the abovesteps, an adhesion layer including a Cr alloy, a soft magnetic layerincluding a CoTaZr-based alloy, a base layer including Ru, aperpendicular magnetic recording layer including a CoCrPt-based alloy, aprotective layer including carbon hydride, and a lubricating layerincluding perfluoropolyether were sequentially deposited on each surfaceof the glass substrate, so that a perpendicular magnetic recording diskwas manufactured. While this configuration shows an example of aconfiguration of the perpendicular magnetic recording disk, the magneticlayer and the like may be configured as an in-plane magnetic disk.

The obtained magnetic disk was confirmed to have no defect found in eachfilm such as a magnetic layer due to a foreign substance. Moreover, themagnetic disk was subjected to a glide test, as a result, hit (aphenomenon that a head scratches a projection on a magnetic disksurface) or crush (a phenomenon that a head collides with the projectionon the magnetic disk surface) was not found. Furthermore, a reproducingtest was performed using a magnetoresistive head, as a result, falseoperation caused by thermal asperity was not found in reproducing.

Example 2

A glass substrate for a magnetic disk was manufactured in the same wayas in the example 1 except for performing a chemical strengthening stepbefore an end face polishing step in the described manufacturing method.Removal depth of the inner circumferential end face in the end facepolishing step was 4 Jim, and inner diameter circularity of the glasssubstrate was 3 after the end face polishing step. A magnetic disk wasmanufactured in the same way as in the example by using the glasssubstrate for a magnetic disk. The magnetic disk was subjected to aglide test, as a result, hit or crush was not found. Furthermore, areproducing test was performed using a magnetoresistive head, as aresult, false operation caused by thermal asperity was not found inreproducing.

While the preferred embodiment of the invention was described withreference to accompanying drawings hereinbefore, it will be appreciatedthat the invention was not limited to such embodiment. It is obviousthat those skilled in the art may think of various alterations ormodifications within a category according to claims, and it can beunderstood that they naturally belong to the technical scope of theinvention.

For example, while only the inner circumference polishing section isused for polishing of the inner circumferential end face in theembodiment, it can be used in combination with another polishingmaterial, for example, a polishing brush. In this case, first, thepolishing brush is used to roughly polish the inner circumferential endface and chamfered portions, and then the inner circumference polishingsection is used for final finishing.

A method of manufacturing a glass substrate for a magnetic diskaccording to the embodiment, in which an inner circumferential end faceof a cylindrical polishing object is polished, the polishing objectincluding a plurality of plate-like (or acceptably disk-like) glasssubstrates stacked on one another, each substrate having an inner holeformed in its center, may be designed such that polishing cloths arepressed in face contact to the inner circumferential end face, then apolishing liquid is supplied between the inner circumferential end faceof the polishing object and an inner circumference polishing section,and then the inner circumference polishing section is turned on its ownaxis so as to polish the relevant inner circumferential end face.

INDUSTRIAL APPLICABILITY

The invention can be applied to a method of manufacturing a glasssubstrate for a magnetic disk, a method of manufacturing a magneticdisk, and a polishing apparatus of the glass substrate for a magneticdisk.

The invention claimed is:
 1. A method of manufacturing a glass substratefor a magnetic disk, including an inner circumference polishing step ofpolishing an inner circumferential end face of a cylindrical polishingobject including a plurality of disk-like glass substrates stacked onone another, each of the disk-like glass substrates having an inner holeformed in its center, wherein: a polishing member, which has a rotationaxis and polishing parts provided around the rotation axis, is insertedinto an inner hole of the polishing object, then the polishing parts areexpanded in a direction perpendicular to the rotation axis of thepolishing member, thereby the polishing parts are elastically pressed tothe inner circumferential end face of the polishing object at evenpressing force and are contacted in face contact to the innercircumferential end face of the polishing object, the polishing partscomprise grinding stones, respectively, and a liquid coolant is suppliedbetween the grinding stones and the inner circumferential end faces ofsaid plurality of disk-like glass substrates, and then, while the centerof the inner hole of each of the disk-like glass substrates and therotation axis of the polishing member correspond during polishing andwhile the polishing parts are simultaneously in face contact of 50% ormore with the inner circumferential end face of the polishing objectduring polishing, at least one of the polishing object and the polishingmember is relatively moved, thereby the inner circumferential end facesof said plurality of disk-like glass substrates are mirror-polished. 2.The method of manufacturing a glass substrate for a magnetic diskaccording to claim 1, wherein: the plurality of polishing parts of thepolishing member are provided in positions reverse to one another. 3.The method of manufacturing a glass substrate for a magnetic diskaccording to claim 1, wherein an outline formed by the outer surfaces ofthe polishing parts is formed on a curved surface having the same radiusas that of the inner circumferential end face of the polishing object.4. The method of manufacturing a glass substrate for a magnetic diskaccording to claim 1, wherein each of the polishing parts has a lengthso that the inner circumferential end faces of all of the disk-likeglass substrates stacked on one another are simultaneouslymirror-polished.
 5. The method of manufacturing a glass substrate for amagnetic disk according to claim 1, wherein the polishing is carried outwith both ends of the rotation axis of the polishing member rotatablysupported or fixed.